One well - known John Williams' work is 'Stoner'. While not strictly a traditional 'romance novel' in the sense of a bodice - ripper, it does explore deep human relationships and love in a very poignant way. The relationship between Stoner and his wife, Edith, is complex and filled with the sort of emotional turmoil that makes for a very engaging read in terms of love and relationships.
I'm not sure if there are many 'John Williams' specifically known only for romance novels. There might be some confusion as the name John Williams is more commonly associated with the famous composer. However, if there are novels by a different John Williams, they may not be widely known in the mainstream romance genre. You could try looking in local libraries or specialized bookstores where they might have a better collection of less - known authors.